Resolve tagged git refs for build cache

[yoshifuminakamura]

Summary

• resolve cached git source refs through heads and tags during build cache restore
• prefer peeled annotated-tag commits, refs/tags/^{}, so cache validation compares the actual built source commit
• add a build cache regression test that stores and restores an annotated-tag build

Tests

• bash scripts/tests/test_build_cache.sh
• bash scripts/tests/test_bk_fetch_source.sh
• shellcheck -S error scripts/build_with_cache.sh scripts/tests/test_build_cache.sh scripts/tests/test_bk_fetch_source.sh
• bash -n scripts/build_with_cache.sh scripts/tests/test_build_cache.sh
• git diff --check